When tiling sub-images of a tiled image, hard edge or soft edge techniques can be applied.
Where hard edge techniques are applied, the projected sub-images are put next to each other. This is often not an optimal solution because the edges between the sub-images are difficult to arrange in perfect alignment and the join may well be noticeable.
Where soft edge techniques are applied, an overlap area is created in which two or more neighbouring sub-images, i.e. at least a first sub-image and second sub-image, are blended so that there is at least a gradual transition from the first sub-image to the second sub-image. Therefore the first sub-image is progressively spatially faded out whilst the second sub-image is progressively spatially faded in. The principle of soft edging is illustrated by means of FIG. 1. In FIG. 1, a first sub-image 2 and a second sub-image 4 are projected by means of a first projector 6 and a second projector 8, respectively, in such a way that an overlap area 10 is created in the image 12 formed by the first sub-image 2 and the second sub-image 4 on the screen, namely in the area which is illuminated by more than one projector, in the present case by the first projector 6 and the second projector 8. In case of a good soft edge, the first and second sub-images 2, 4 are processed, optically or electrically, in such a way that the sum of the light intensity in the overlap area 10 is equal to the average intensity outside the overlap area 10, and in such a way that the contribution of the first projector 6 to this total intensity in the image 12 changes gradually from maximum to zero from a first side of the overlap area 10 to a second side of the overlap area 10, while the contribution of the second projector 8 to this total intensity changes gradually from zero to maximum from the first side of the overlap area 10 to the second side of the overlap area 10.
Two types of soft edge techniques or blending techniques are known: electrical blending and optical blending.
The use of electrical edge blending is known in the industry and is widely used in cathode ray tubes (CRT), digital light processing displays (DLP™), liquid crystal displays (LCD) and other projection display technologies.
In U.S. Pat. No. 4,974,073 a seamless video display is generated from multiple discrete video images by overlapping the images and ramping the image brightness in the overlaps. The resulting composite image is of uniform brightness and has no seams between the images of which it is composed.
U.S. Pat. No. 5,136,390 describes a method and apparatus for establishing consistent image brightness, especially for a multiple video image seamless display. A set of smoothing factors is stored in a memory, one for each detail element of each image. The method comprises applying a predetermined set of smoothing factors to the brightness components of the detail elements of the two signals, projecting the images as modified by the smoothing factors onto a display, modifying selected smoothing factors in response to the appearance of the projected display, and finally, storing a representation of the smoothing factor modifications. This allows a seamless multiple video image display to appear more consistent and uniform in brightness than a conventional single video image display.
The advantages of electrical blending are known: real time control, dynamically changeable based upon imagery or changes in the system configuration, flexible attenuation curves of any type can easily and simply be defined.
However, in the case of modern light modulators such as DLP™ and LCD, maximum attenuation in the blend region of an electronic blending system does not produce an entirely black image due to light leakage inherent in the modulator technology. This means that black and near-black imagery cannot be successfully blended using only electronic means. This problem is known as “double black” or “non-zero black level problem”: the combination of the two light leakages in the overlapping blend region will lead to a black level intensity that is twice the level of the black level intensity in non-overlapping areas. Such a region is easily detected when black or near-black images are displayed, as the eye is very sensitive to abrupt transitions in brightness as occurs in an uncompensated blend region.
It is known, for example from US-2002/0057361 to compensate for this effect by boosting the minimum black level of the video signal in non-overlapped areas without affecting the bright portion of the video signal in order to create uniform black level. Although this method can be used, it results in a loss in system contrast.
It is also known to use optical masks either with hard edges, dither patterns, or gradient patterns of some kind and by placing such devices either internally or externally to the projectors to cause smooth transitions in blend regions to produce a satisfactory blend. Such systems are described for example in WO 95/25292 and WO 01/41455.
Optical blending systems do not suffer from the “double black” problem and offer an acceptable blend at all brightness levels from white to black. Nevertheless, the designing, manufacturing and alignment of the optical masks in a projector system needs to be done very carefully, and consequently is time consuming and expensive. Furthermore, modifications to the blending shape are difficult and time consuming and due to their inherently fixed nature, changes to the blend regions for the purposes of changes in display configuration are not possible without articulated mechanical systems. The advantage of an optical blending system is that it does not suffer from the “double black” problem and can therefore produce an acceptable blend at all brightness levels from white to black when property designed and installed.
U.S. Pat. No. 6,570,623 describes the use of a blending frame to control the degree of intensity reduction in the overlapping region of two adjacent images projected by two different projectors in a video wall system. It furthermore comprises the use of digital compensation to fine-tune the optical blending results by digitally altering the image source using a camera-based feedback loop. Nevertheless, the blending method described in U.S. Pat. No. 6,570,623 still suffers from the problems inherent to optical blending, e.g. difficulties with design, manufacture and alignment of optical masks and difficulties to maintain and reconfigure the system without the need of mechanical articulation devices.